The present invention relates to methods for the preparation of multi-layered coated articles and to the resultant coated articles.
In the coating of substrates such as, for example, automobiles, where a coating is desired which provides not only a lustrous appearance but also good corrosion protection, it has been well appreciated to provide the substrate with several coating layers in order to achieve the desired effect. Therefore, typically a pigmented coating composition is first applied to the substrate followed by a transparent topcoat. By transparent is meant a clear coating, that is, one which does not contain pigmentation or contains only transparent pigments. Such a coating system is commonly referred to as a "clear-over-color" or "color plus clear".
In the area of automotive coatings, metallic pigments such as aluminum flake are generally utilized to obtain the glossy lustrous appearance which is characteristically sought. In order to achieve the preferred appearance of such metallic coating compositions it is very important that the metallic pigment orient such that it is parallel to the coated surface. The alignment of the pigment in this fashion provides for the most desirable appearance, especially with respect to the "flop" of the coating. By "flop" is meant the visual change in brightness or lightness of the metallic aluminum flake with a change in viewing angle, that is, a change of from 90 to 180 degrees. The greater the visual change from light to dark appearance, the better the flop. The flop accentuates the lines and curves of an automobile; therefore, it is very important in achieving the sought after appearance of the coating.
In addition to "flop", another very important feature required of color plus clear systems is good "strike in" ("soak-in") resistance of the base coat. By "strike in" resistance is meant the ability of the basecoat composition of a multi-coat system to resist attack by the solvents in the topcoat composition. The topcoat "striking in" to the basecoat is particularly undesirable since it adversely affects alignment of the metallic pigment and reduces or eliminates the "flop" and the gloss of the topcoat. The strike in is a problem because the automobile manufacturers generally wish to apply the topcoat composition to the basecoat composition in a color plus clear system by a "wet-on-wet" technique. By this is meant that the basecoat composition is applied to the substrate followed by the topcoat composition and then a single baking step is utilized to cure the composite coating. The problem of the topcoat "striking-in" to the basecoat is particularly pronounced with high solids clear topcoating compositions which are formulated from low molecular weight polymers.
Aqueous based coating compositions are particularly desirable for use in the basecoat of multi-layered coating systems because they provide very bright coatings with exceptional flop. Aqueous polyurethane dispersions are particularly desirable as binders in such multi-coat systems. For example, U.S. Pat. No. 4,489,135 to Drexler et al discloses a process for preparing a multi-layer coating wherein the coating agent for the base layer contains as a binder an aqueous polyurethane dispersion which has been prepared from among other components, a linear polyether diol and/or polyester diol having terminal hydroxyl groups and having a specified molecular weight. Aqueous polyurethane dispersions such as those of Drexler et al which are prepared from linear diols quite often have difficulties associated with them. For example the polyurethane dispersion typically requires the presence of an external emulsifying agent or alternatively, the incorporation of hydrophilic monomers into the backbone of the polyurethane in order to achieve the requisite stability of the dispersion. Typically the strike in resistance of coating compositions prepared from such dispersions is not optimum.